1. Field of the Invention
This invention relates to a switching power supply unit such as a DC-DC converter, and more particularly to a switching power supply unit that is capable of reducing noise and energy loss.
2. Description of Related Art
As a conventional power supply, a switching regulator is widely used which is constructed such that a switching device serially-connected to a transformer is turned on/off to convert a direct-current voltage into an alternating-current voltage, which is then applied to a primary winding of the transformer to cause a secondary winding thereof to generate an alternating-current voltage stepped down according to a turn ratio thereof, and the alternating-current voltage is rectified and smoothed to obtain a direct-current voltage.
A one-chip type forward converter as shown in FIG. 12 is known as an example of the switching regulator.
The one-chip type forward converter shown in FIG. 12 is constructed such that direct-current voltage Vin obtained by rectifying and smoothing a commercial alternating-current power supply input by a rectifier circuit, not shown, is applied to a serial circuit that is comprised of an insulating transformer T and a semiconductor switching device Q1.
A reset circuit that is comprised of a diode D1, a capacitor C1 and a resistance R1 is connected to both ends of a primary winding N1 of the insulating transformer T.
A pair of a rectifier diode Ds1 and a flywheel diode Ds2 is connected to a secondary winding N2 of the insulating transformer T. The respective cathodes of the diodes Ds1 and Ds2 are connected to each other and are connected to the positive end of a smoothing capacitor C2 via a choke coil Lo. The negative end of the capacitor C2 is connected to an anode of the diode Ds2 and the positive end of the secondary winding N2. These circuits constitute a secondary rectifier/smoothing circuit.
There will now be outlined the operations of the forward converter in FIG. 12.
The semiconductor switching device Q1 is turned on/off in synchronism with a drive signal. This drive signal is generated by a conventional PWM control circuit (not shown) that controls the ON/OFF period ratio of the semiconductor switching device Q1 while monitoring a direct-current output voltage from the converter so that the direct-current output voltage becomes equal to a desired constant voltage.
Under the control of the PWM control circuit, the semiconductor switching device Q1 is switched at a substantially higher frequency than that of a commercial alternating-current power supply (50 Hz or 60 Hz). This causes the direct-current input voltage Vin to be applied to the primary winding N1 of the insulating transformer T only while the semiconductor switching device Q1 is ON, and causes the secondary winding N2 of the insulating transformer T to generate an alternating-current voltage according to a turn ratio of the transformer T. The generated alternating-current voltage is rectified by the diodes Ds1 and Ds2, and smoothed by the choke coil Lo and the smoothing capacitor C2, and a direct-current output of a predetermined voltage is acquired between the terminals of the smoothing capacitors C2.
When the semiconductor switching device Q1 is turned off, excitation energy accumulated in the insulating transformer T during the ON period is converted into thermal energy and consumed by the resistance R1 in the reset circuit comprised of the diode D1, the capacitor C1 and the resistance R1, so that the excitation energy is reset. This enables absorption of a surge voltage.
In this case, the maximum duty of the PWM control circuit is set to be not greater than 50% in terms of the time required for resetting the excitation energy accumulated in the transformer T.
Thus, in the case of the forward converter, the insulating transformer T becomes saturated unless the excitation energy therein is reset.
The CRD circuit serving as a conventional reset circuit causes its resistance to consume the excitation energy, and this results in energy loss.
The switching power supply unit that acquires a direct-current output by interrupting a voltage by the switching device causes power loss since changes in current and voltage overlap with each other in the turn-on period and turn-off period of the switching device (see FIGS. 13A to 13D).
To eliminate such energy loss, a partial resonance type power supply has been proposed in which a capacitor is connected in parallel with a switching device, and when the switching device is turned off, a surge voltage is absorbed by resonance and a terminal voltage across the capacitor is gradually raised so as to reduce switching loss such as the above-mentioned energy loss and power loss, and then energy accumulated in the capacitor is regenerated to an input side.
In such a partial resonance type power supply, a capacitor Ccv is connected in parallel with a main switching device Q1 via an auxiliary switching device Q2, and the ON/OFF timing of the auxiliary switching device Q2 is shifted from that of the main switching device Q1 as shown in FIG. 3 such that excitation energy is once accumulated in the capacitor Ccv and then the excitation energy is regenerated to an input side, and also the main switching device Q1 is switched after the terminal voltage across the main switching device Q1 is lowered to zero.
The partial resonance type power supply has the following advantages, for example: (1) it can be controlled by PWM; (2) the zero-voltage switching enables a reduction in switching loss and noise; and (3) the regeneration of excitation energy reduces reactive power. In particular, the partial resonance type power supply controlled by PWM has the following advantages: (4) it is possible to use an inexpensive PWM control IC; and (5) it is easy to design the transformer and reduce noise since the driving frequency is fixed.
To reduce the loss in the regeneration of electric current in the partial resonance type power supply, a variety of methods have been proposed: e.g. an inductance is inserted in parallel with the secondary winding, or a tertiary winding is used.
With any of these methods, however, if a forward converter is used as the partial resonance type power supply, a rectifier diode and a flywheel diode are made to conduct at the same time for a long period of time in a rectifier diode section on the secondary side as shown in FIG. 14. Therefore, at the same input voltage, the power transmission efficiency is deteriorated compared with the conventional power supply unit although the loss is reduced.
If the conventional forward converter controlled by PWM is modified directly into a partial resonance type forward converter, since the maximum duty of the main switching device is not greater than 50% due to the reset period, if the load current is increased, regenerative current flowing to the input side during the OFF period is larger than current flowing during the ON period during which power is transmitted to the secondary side. This may cause excitation of the insulating transformer T in the negative direction (negative excitation).
Even if the restraint on the maximum duty can be released, since the timing of driving the auxiliary switching device is determined according to the time period that is required for discharging regenerative current in a light load state, in the case of a power supply unit installed in an apparatus such as a copying machine whose load fluctuates in a very wide range because most of the load is applied by a motor, in a heavy load state regenerative current during the OFF period is larger than the current flowing during the ON period during which power is transmitted to the secondary side as in the case where the maximum duty is restrained. This results in the negative excitation of the transformer T. Therefore, the resonance has to be made in a limited load range. Consequently, the effects of low loss and low noise obtained by zero-voltage switching cannot be achieved in a light load state.
The degraded power transmission efficiency and the negative excitation of the insulating transformer T are caused by the maximum duty being set to 50% or less in the case where the conventional PWM controlled forward converter circuit is used.
If excitation energy is regenerated to the input side, the value of regenerated current varies according to the load conditions. Thus, in a period (dead time) during which the main and auxiliary switching devices are OFF, the time required for completely discharging the capacitor storing excitation energy therein by resonance is likely to be long in a light load state and short in a heavy load state.
It is therefore an object of the present invention to provide a switching power supply unit that is capable of performing zero-voltage switching in the entire load region to reduce switching loss and noise and improve the voltage transmission efficiency.
To attain the above object, the present invention provides a switching power supply unit comprising a main switching device, an auxiliary switching device for controlling resonance so as to prevent power loss accompanying a switching action of the main switching device, and varying means for varying an OFF period during which both the main switching device and the auxiliary switching device are OFF, according to power consumption by a load.
Preferably, the varying means increases the OFF period as power consumption by the load is smaller and decreases the OFF period as the power consumption by the load is larger.
In a preferred embodiment of the present invention, there is provided a switching power supply unit of a PWM-controlled forward converter type comprising an input power supply, a transformer having a primary winding connected in series to the input power supply, a secondary winding, and a tertiary winding, a resonance inductor connected in series to the input power supply, main and auxiliary switching devices of a field effect type, a rectifier diode connected to the secondary winding, a flywheel diode connected to the secondary winding, an inductor connected to the secondary winding a first capacitor formed of a parasitic or independent capacitor connected in parallel between a drain and a source of the main switching device, a first diode connected in anti-parallel between the drain and source of the main switching device, a second capacitor connected in parallel with the main switching device via the auxiliary switching device, the second capacitor having a larger capacitance than the first capacitor, a third capacitor formed of a parasitic or independent capacitor connected in parallel between a drain and a source of the auxiliary switching device, a second diode connected in anti-parallel between the drain and source of the auxiliary switching device, a third diode connected to the tertiary winding, a PWM control circuit that generates a first control signal for turning on/off the main switching device, and a second control signal for turning on/off the auxiliary switching device so as to give a predetermined dead time to an ON-OFF period of the first control signal, and a control circuit that varies timing of the second control signal according to a power consumption level of a load on an apparatus in which the switching power supply unit is installed, and the tertiary winding has one end thereof connected to the secondary winding such that the tertiary winding is identical in magnetic polarity with the secondary winding, and the tertiary winding has another end thereof connected to the third diode such that excitation energy accumulated in the transformer is outputted to a positive pole of an output side of the switching power supply unit while allowing adjustment of regenerated current at an input side of the switching power supply unit when the main switching device is turned off.
Typically, the apparatus in which the switching power supply unit is installed is a copying machine.
According to the arrangement of the present invention, the switching power supply unit having a partial resonance function using the resonance control auxiliary switching device provides the optimum dead time control according to the power consumption by the load. This improves the power supply efficiency.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.